The present invention generally relates to fluid sensing devices and methods of using such devices. More particularly, this invention relates to a micromachined fluid sensing device capable of measuring properties of a fluid in a fluid system that exceeds the internal flow capacity of the device.
Processes and designs for resonant mass flow and density sensors using silicon micromachining techniques are disclosed in commonly-assigned U.S. Pat. Nos. 6,477,901, 6,647,778, 7,228,735 and 7,263,882, as well as GB 2,221,302A, and WO2007/147786 A1. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. In the processes disclosed by U.S. Pat. No. 6,477,901 to Tadigadapa et al. and U.S. Pat. No. 6,647,778 to Sparks, wafer bonding and silicon etching techniques are used to produce microelectromechanical systems (MEMS) comprising one or more suspended silicon tubes on a wafer. The tube is vibrated at resonance, by which the flow rate and density of a fluid flowing through the tube can be determined.
Sensors of the type taught by the above-noted U.S. patents have found use in a variety of applications. A notable advantage of these sensors is the extremely miniaturized scale to which they can be fabricated, which among other things enables the sensors to precisely analyze very small quantities of fluids. However, in certain applications where relatively large volume flow rates exist, the limited flow capacity of these miniaturized sensors can be inadequate. Nonlimiting examples include industrial applications in which the flow of petrochemicals, gases, water, air, and other liquids flow through relative large pipes that can be a meter or more in diameter. Other nonlimiting examples include fluid flows in automotive and aerospace applications, including air intake, petrochemical fuels, hydrogen, alcohols, etc. Existing flow sensors typically utilize hot-wire and drag-force technology. However, it would be desirable if sensors of the type taught by Tadigadapa et al. and Sparks could be adapted for relatively high flow applications without necessitating an increase in sensor size.